Vitreous enamel compositions for aluminum and its alloys



VTTREOUS ENAMEL COMPOSITIONS FOR AL AND ITS ALLOYS Application February6, 1956. Serial No. 563,811

11- Claims. 01. 10648) N Drawing.

This invention relates to vitreous enamel compositions forproducingprotective and' decorative coatings on aluminum and aluminum alloys, andmore particularly to vitreous enamelcompositions based upon zinc oxideas a flux and insolubilizing agent, and upon antimony trioxide topromote adherence of the compositions to the aluminum and aluminumalloys.

This application is a continuation-in-part, of application Serial No.451,881, filed August 24, 1954, and now abandoned.

Aluminum and its alloys have many properties which suit themfor avariety ofuses, but the exposed metals cannot be used where the surfaceof the aluminum is subject to chemical attack. Aluminum is very reactiveand will corrode rapidly in contact with many materials, such as seawater and mild alkaline detergents.

Efforts have been made heretofore to overcome this difiiculty. bycoating the aluminum or aluminum alloy surface with a protectivecovering. Paints and lacquers have been' used, but these coatings arenot permanent. Vitreous enamels have been tried, but it is difiicult toformulate a vitreous enamel which will adhere to aluminum.

The coating of aluminum and its alloys with vitreous enamels presentsmany problems; Aluminum articles are known to deform and lose tensilestrength at temperatures in excess of approximately 1000 F. Mostvitreous enamels used for enameling cast iron and steel are intended tomature at temperatures of the order of 1300 to 1500 F., and cannot ofcourse be used for aluminum. Many .enamel compositions which have beenavailable 'and which mature at low enough temperatures do not adheretothe aluminum very well, and tend to flake or spall spontaneously from.the aluminum after firing. The same result may occur after the aluminumhas been exposed to weathering or to highly moisture laden atmosphere.

Various types of enamel compositions said to be particularly suitablefor aluminum have been proposed.

The Warga Patents Nos..1,23.0,958 and 1,346,475 disclose enamels whichdo not mature to a glossy surface at temperatures low. enough to avoiddeterioration of the aluminum article, and still possess resistance toweathering and other chemical action as well as adherence to the metal.

The Deyrup PatentsNos. 2,467,114, 2,544,139 and 2,653,877 describevitreous enamel compositions intended for use with aluminum anditsalloys based upon plumbous oxide PbO, silicon dioxide SiO and lithiumoxide LiO' as'essential ingredients, and sodium oxide Na O, potassiumoxide K 0, and titanium dioxide T102, as optional ingredients, inspecified proportions. The lead oxide is used to attain the required lowmaturing temperature. However, certain trade restrictions make themanufacture and use of lead in vitreous enamel compositionsobjectionable, because of its toxicity. Also, lithium oxide isrelatively expensive.

U.S. Patent No. 2,660,531 to Fraser and Cianchi describes enamelcompositions based upon alkaline earth oxides as essential ingredients,together with a small amount of boric oxide and, optionally, .ofaluminum oxide; These avoid the use of lead oxide. However, the 5alkaline earth metal oxides are quite reactive, being strong basessparsely soluble in water, so that the acid resistance of theseenamelsis not as good as might be desired. Also, they show poor bonding toaluminum.

It has now been determined that neither lead oxide nor lithium oxide noralkaline earth oxides are requisite for t the formulation of vitreousenamels useful in coating aluminum'and aluminum rich alloys.Compositions can be formulated based upon zinc oxide ZnO and antimonytrioxide Sb O and the vitreous enamel compositions of the invention canbe applied to aluminum metal at temperatures within the range from 900to 1000 F., low enough to avoid damage to the metal but high enough toprovide'hard, smooth, glossy, durable coatings which do not tend tospall or flake from; the metal base, and which 0 provide excellentprotection against attack by the usual chemical agents. p

The compositions of theinvention contain as essential ingredients borontrioxide B 0 as aflux and a glassforming agent, silicon dioxide SiO as aglass-forming agent,-titanium dioxide TiO as an opacifying, glassforming and insolubilizing agent, zinc oxide ZnO as a flux andinsolubilizing agent, both sodium and potassium oxides Na O and K 0 asfluxes, because their eutectic mixture gives a lower fusion temperaturethan either alone, and antimony trioxide Sb O as an adhesionpromotingagent to aid in'adhering the enamel compositions to the metal. Thesecomponents are used in the following proportions:

Range 1n Range in Weight Mole (Percent) (Percent) ZnO 0. 5-10. 0 0.75-9. 0 K20 6. 0-23. 0 6. 0-22. 0 Si 15. 0-35. 0 14. 0-35. 0 1. 0-16. 00. 75 -4. 0

The above-named ingredients can be supplied to the enamel compositions.as any convenient, compound of the named elements, which may be theoxide itself, if available,-or another compound easily decomposed orreduced under the enamel firing conditions to form the oxide. Thus,potassium carbonate and sodium carbonate are convenient sources ofpotassium oxide and sodlum oxide, respectively. Boric acid is aconvenient source of boron trioxide. The other named oxides are readilyavailable, and would usually be used.

In order to formulate compositions useful for special purposes, othersubstances can be used in partial substltution for the named essentialingredients. The substitutions are always on a mole-for-mole basis.However, for convenience, the amounts in the following discussion aregiven in weight percents.

The antimony trioxide promotes adherence to the aluminum because it isan easily reduceable oxide, that is, it is reduced by'aluminum at thefiring temperatures. Other aluminum-reduceable oxides can be usedpartially to replace the antimony trioxide. In this category are leadoxide PbO, which also acts as a flux and can be used in amounts up toabout 25%, cadmium oxide CdO, in amounts up to 7.5%, cupric oxide CuO,in amounts up to 7%, cobaltous oxide C00, in amounts up to 13%,nickelous oxide NiO, in amounts up to 13 ferrous oxide FeO, in amountsup to 13%, stannic oxideSnO in amounts up to 8%, and molybdic oxideM00;, in amounts up to 6%.

Patented Oct. 20, 1.959

Cadmium oxide, cupric oxide, cobaltous oxide, nickelous oxide andferrous oxide, because they act as fluxes, can be used in the amountsstated to partially replace the zinc oxide.

In'order to promote lower fusability and increase insolubility of theglasses, up to 5% lithium oxide Li O can be used as a flux in partialreplacement for sodium and potassium oxides. This material can besupplied as lithium carbonate.

Cobaltous oxide can be added to the batch to provide a transparent blueenamel. Cadmium oxide will make the enamel more suitable for red colors,using cadmium sulfoselenide pigments.

In the usual enamel practice the raw materials constituting the enamelare first smelted at temperatures above 1900" F., and the resultingglass is quenched. This frit is then milled with suitable electrolytesand floating agents, if desired, together with pigments, if necessary,and sprayed on the metalware. After partially drying the ware is firedat a temperature high enough to produce fusion of the enamel, but lowenough to avoid damage to the metal, and the familiar glossy, hard,chemically resistant, vitreous enamel finish results.

Frits can be prepared in accordance with the invention by weighing out abatch of the ingredients to supply the stated oxides in the requiredamounts, mixing and heating the batch until it is completely melted toform a homogeneous glass. Temperatures within the range from 1900 to2200 F. are suitable. The molten glass is fn'tted by running it intowater, in which it shatters into small pieces, forming the frit. Thefrit then is dried at a low temperature, and ground either dry or withwater or other suitable liquid. During the grinding it may be convenientto include with the frit, pigments such as titanium dioxide, in amountsup to 30% by weight of the total enamel composition. Pigments, ofcourse, can be omitted if a clear, colorless, vitreous coating isdesired.

The powdered enamel then desirably is dispersed or suspended in asuitable liquid, which may be applied to the aluminum by any of the wellknown procedures, including spraying, dipping, brushing, stenciling,etc. Alcohol-water dispersions have been found to be satisfactory whenprepared and applied by skilled personnel. When a dispersion of thistype is not carefully handled, however, the resultant enamel coating hasa tendency to tear and crawl. This problem can be avoided by using anantitearing vehicle such as sodium metasilicate or sodium polysilicatein conjunction with borax, boric acid, sodium nitrite or sodiumtetraphosphate in water suspensions. Any manner which will provide auniform and smooth coating of the composition before firing can be used.The surface of the aluminum rich metal which is to receive the enamel isfirst prepared for enamelling in conventional manner, for example, bypretreating this surface with a chromate, perchlorate or permanganatesolution in aqueous media and subsequently heat-treating this surface.The thickness of the coat will of course depend upon the thickness ofthe final enamel desired, and will vary Within the range from to g. persquare foot per coat. Coating thicknesses of 0.002 to 0.003 inch arepreferred. Generally, a ground coat and one finish coat are applied, butone coat may be suflicient, and more coats could of course be added ifdesired, the furnace firing being repeated after application of eachcoat.

The coated metal then is fired in a suitable furnace for a time and at atemperature sufiicient to fuse the enamel to a continuous glassycoating. The temperature should be high enough to melt the enamel, butnot high enough to injure the metal. Temperatures within the range from900 to 1000 F. are entirely satisfactory, and usually the enamel hasmatured in a few minutes, rarely more than ten minutes being necessary.

The following examples illustrate enamel compositions coming within theinvention:

4 EXAMPLES 1 AND 2 These formulations produce clear vitreous enamels:

Table I Batch Weight Percent Compositions Example 1 Example 2 Zinc Oxide(ZnO) 6. 2 5. 0 Potassium Carbonate (K2003) 16. 6 19.8 Sodium Carbonate(N21200:) 25.4 21. 7 Silica (SiOz) 22. 0 14. 7 Titanium Dioxide (T102)22.0 14. 7 Boric Acid (HgBOz) 4. 6 15.2 Antimony Trioxide ($13203) 3. 28.9

Table II Calculated Weight Percent Compositions Example 1 Example 2Table III Empirical Molecular Formulae 1 Example 1 Example 2 ZnO 0.1750.15 0. 275 0. 35 0. 55 0.50 0. 84 0.60 0.63 0.45 0. 084 0. 30 0. 0250.075

1 Molecular percent is obtained by dividing each equivalent by the sumof equivalents and multlplying by 100.

suspension was then sprayed on an aluminum sheet to give a coating ofabout 15 grams per square foot. This coating was fired at 950 F. forseven minutes. A second coating was applied of about 15 grams per squarefoot, and fired at the'same temperature. A hard, chemically resistantvitreous enamel which was clear and colorless was obtained.

In order to test the spalling resistance of these enamels, the enamelledsheets were immersed in a 5% aqueous solution of ammonium chloride for96 hours. Spalling did not take place.

The'enamelled sheets were spot tested for acid resistance by placing afew drops of 10% aqueous citric acid on a portion of the sheet, coveringthis portion with a watch glass. The glass was removed after a fifteenminute interval. No surface changes in the enamels had occurred.

EXAMPLES 3 TO 7 The following compositions yield clear vitreous enamelsand illustrate the use of other oxides .with zinc oxide and the use ofother aluminum-reduceable oxides with antimony trioxide.

pass. 325 mesh with 3 parts sodium silicate, 3 parts borax and 35 partswater per 100' parts frit. The suspension thus produced wassprayed on analuminum sheet to produce a coating weighing about 20 grams per squarefoot. This was fired at .1000" F. A second coating of about 20 grams persquare foot was applied and fired at the same temperature. A very white,glossy, hard, chemically resistant vitreous enamel finish was obtained.

EXAMPLES 10 TO 14 These compositions show how lead oxide can be usedwith the zinc oxide to make high gloss, good adhering compositions.

Table X Batch Weight Percent Compositions Example Nos 10 11 12 1a 14Zinc Oxide (ZnO) 1.6 0. 7 0.8 2. 5 2.5 Potassium Carbonate (K1003) 13. 713. 2 15.0 15.6 16. 4 Sodium Carbonate (NazCOg) 16. 8 l9. 2 21. 9 24.025.0 Silica 19. 19. 2 19. 8 20. 8 21. 7 Titanium Dioxide (T101) 19.019.2 19. 8 20. 8 21.7 Boric Acid (HaBOa) 2.0 3. 9 4. 1 4. 3 4. 4Antimony Trloxide (SbzO 3. 0 2.8 2. 9 3. 0 3.1 Red Lead (PbaOi) 18.121.8 15.7 9.4 2.5 Cadmium Carbonate (CdCOz) 6.8

Table XI Calculated Weight Percent Compositions Examples Nos 10 11 12 1314 Table XII Empirical Molecular Formulae 1 Examples Nos 10 11 12 13 141 Molecular percent is obtained by dividing each equivalent by the sumof equivalents and multiplying by 100.

The above compositions were smelted at 2100 F.,

'fritted by running into cold water, dried and ground to produceacoating weighing about 20 grams per square foot. This was fired at 1000F. A second coatingof about 15 grams per square foot was applied andfired at the same temperature. A clear, colorless, hard, chemicallyresistant vitreous enamel finish was obtained.

The fritted enamel compositions in accordance with the invention areparticularly useful in making chalkboard enamel compositions. Milledchalkboard enamel slips are prepared by incorporating in the ball-millor grinding batchwith a fritted enamel composition of the inventionappropriate amounts of abrasive or matting agents, ceramic colorpigments for coloring, boric acid and sodium metasilicate asanti-tearing and setting-up compounds, and water. The batch is groundvery fine, so

that less than 0.25 g. of residue remains when 50 ml.

of ground slip are washed through a 325 mesh screen.

The amounts of these ingredients are not critical, and would be used toformulate an enamel of the desired hardness and other use properties.

Usually, the amount of the abrasive or matting agent or mixture thereofwill lie within the range from about 20 to about 60%, the. ceramic colorpigment or mixture thereof will constitute from about 0 to about 40% andthe anti-tearing and setting-up compound will constitute from about 5 toabout 12%.

About half of the chalkboard enamel composition will constitute the fritof the invention, but the amount of this, too, may be widely varied, andusually would lie within the range from about 35 to about 70%. Thevehicle is water, and this constitutes the remainder of the composition.The amounts of the ingredients are selected from the above ranges togive, after firing, a chalkboard having high scratch resistance, thedesired matte finish, and perfect writing, erasing and wearingproperties, while retaining a good enamel to aluminum or alumifaces inaccordance with the invention containing alumina in the amountsspecified show scratch hardnesses ranging from 8 to 9. Other matting orabrasive agents can be used with good results, but at the cost of alower scratch resistance. In this group are chromium oxide, titaniumdioxide, zirconium oxide, zirconium silicate, tin oxide, antimony oxideand silica. These are used in various combinations of one or more withthe alumina, or they can be used without alumina in various combinationsof two or more, and in various proportions, within the stated range,depending upon the effect and the surface hardness required.

The colored ceramic pigments which can be used include the conventionalceramic pigments. The preferred anti-tearing compounds are boric acidand sodium metasilicate, but other compounds known to impart tearresistance and improvesetting-up of the enamels can be used. Furtherdetails need not be given as to these components, inasmuch as they areconventional in enamel mill batches.

The chalkboard enamel slips of the invention usually are applied byspraying to the surface of the aluminum, aluminum alloy or aluminum.coated steel to be coated. Other conventional coating methods can beused, such as by dipping or spraying. They can also be used as coveringcoats over ground coat-enamelled steel. They are fired wet aftercoating, or after air-drying until almost dry, or after drying rapidlyat temperatures above 350 F. If dried at low temperatures, of the orderof to 230 F., tearing or parting of the coat may result. Chalkboardenamel coatings are fired in the range from about 950 to about 1050 F.,according'to the frit used and the makeup of the mill batch.

The following are examples of typical chalkboard enamel mill batches,and of chalkboards prepared therefrom.

Table XIII [Weight percent compositions] The components outlined in thetable were blended and ground in a ball mill to such a fineness thatless than 0.25 g. of residue remained when 50 ml. of the suspension waswashed through a. 325 mesh screen. The compositions then were sprayed onone surface of an aluminum-coated steel bearing an aluminum coatingapproximately 1 mil thick. These plates were fired wet at 1000 F. Theresulting chalkboard surface was light green, had a scratch hardness onMohs scale of above 8, excellent scratch resistance, and a good mattefinish, with perfect writing, erasing and wearing properties.

The enamels of the invention can be used on aluminum and aluminum alloysurfaces, including articles formed of aluminum and aluminum andaluminum-alloy surfaces coated upon other supports, such as aluminizedsteel, which bears a continuous aluminum coating averaging from one toseveral mils thick on its surface. Typical aluminum and aluminum alloysavailable commercially which can be coated with the enamels of theinvention are Alcoa 2S, Alcoa 38 (aluminum alloy containing 1.2%manganese), Alcoa 52S (aluminum alloy containing 0.25% chromium and 2.5%magnesium), Alcoa 53S (aluminum alloy containing 1.3% magnesium, 0.7%sili con and 0.25% chromium), Alcoa 43 (aluminum alloy containing 5%silicon), Alcoa B214 (aluminum alloy containing 3.8% magnesium and 1.8%silicon), Alcoa 220T4 (aluminum alloy containing magnesium) Aluminum99+, Aluminum 9899, 40 alloy (aluminum alloy containing 4% copper) 40Nalloy (aluminum alloy containing 4% magnesium) Red X10 (aluminum alloycontainnig 10% silicon, 1.5% copper, 0.6% manganese, and 0.5%magnesium), and Y alloy (aluminum alloy containing 4% copper, 1.5%nickel, and 1.5% magnesium). A typical aluminized steel is Armco, whichhas an aluminum coating one mil thick.

The enamels of the invention have extremely low fusion points, and yetmaintain a degree of resistance to chemical agents, especially acids,which hitherto has been believed possible only with enamel compositionshaving a fusion point of at least 300 F. higher. In addition to theirlow fusion point, the enamels of the invention have high resistivity tochemical agents. They will adhere to the aluminum or aluminum alloysurface even when the coated surfaces are exposed to thermal shock or tomechanical flexing. These properties are obtainable with relativelythick enamel layers, thick enough to provide freedom from pores andother imperfections. It is thought that the antimony trioxide, acomponent of the enamel, becomes chemically bonded to the aluminum, orto the aluminum oxide film on the surface with the aluminum, or .perhapsalloyed with the aluminum if it is reduced to the metal during firing,and this is believed to 10 be in part responsible for their adhesion tothe aluminum surface despite flexing and thermal shock. However, this isonly a theory; it has not been verified as yet by experimental evidence.

Because the enamels of the invention are leadefree or have a very lowlead content, they are nontoxic or at least far less toxic than thelead-based enamels heretofore available. Thus, they are far lesshazardous to use in the shop and on ware. The low lead content makespossible the production of far better white and colored enamels. Theenamels of the invention have a high expansion coeflicient, more closelyapproximating that of aluminum and aluminum alloys, so that the chanceof spelling is greatly reduced; f

Because the enamels of the invention are free from alkaline earth metalcompounds, they have superior acid resistance and the bonding toaluminum is greatly improved.

All parts and percentages in the specification and claims are by weight,unless otherwise indicated.

I claim:

1. A vitreous enamel composition especially adapted for use as aprotective or decorative coating for aluminum and aluminum base alloysand adapted to be fired at a temperature within the range of from about900 to about 1000 F. consisting essentially of from about 0.5 to about10% zinc oxide, from about 5 to about 25% potassium oxide, from about 5to about 25% sodium oxide, from 0 to about 5% lithium oxide, from 15 toabout 30% silica, from about 15 to about 35% titanium dioxide, fromabout 1 to about 15% boric oxide, from 0 to about 25% plumbous oxide andfrom about 3 to about 13% antimony trioxide, the level of combinedsodium oxide, potassium oxide and lithium oxide coming within the rangeof from about 22.5 to about 37.5%, and the said weight percentages beingbased upon the composition of the enamel after firing.

2. A composition in accordance with claim 1 which contains up to about7.5% cadmium oxide as a second flux supplementing the zinc oxide.

3. A composition in accordance with claim 1 which contains up to about7% cupric oxide supplementing the adhesion-promoting antimony trioxide.

4. A composition in accordance with claim 1 which contains up to about13% total of a mixture of cobaltous oxide, nickelous oxide and ferrousoxide supplementing the adhesion-promoting antimony trioxide.

5. A composition in accordance with claim 1 which contains up to 8%stannic oxide supplementing the adhesion-promoting antimony trioxide.

6. A composition in accordance with claim 1 which contains up to 6%molybdic oxide supplementing the adhesion-promoting antimony trioxide.

7. Aluminum metal coated with a vitreous enamel in accordance with claim1.

8. Aluminum alloy coated with a vitreous enamel in accordance with claim1.

9. A milled chalkboard enamel composition comprising from about 65 toabout 30% of water and from about 35 to about 70% of a fritted vitreousenamel composition in accordance to claim 1, from 20 to 60% of anabrasive, from 5 to 12% of an anti-tearing compound selected from thegroup consisting of boric acid and sodium metasilieate, and up to 40% ofa colored ceramic pigment.

10. A chalkboard comprising aluminum metal coated with a chalkboardenamel composition in accordance with claim 9.

11. A chalkboard comprising aluminum alloy coated with a chalkboardenamel composition in accordance with claim 9.

12.. A chalkboard comprising aluminum-coated steel coated with achalkboard enamel composition in accordance with claim 9.

13. A chalkboard enamel composition ready for milling with watercomprising from about 35 to about 70% of a fritted vitreous enamelcomposition in accordance with claim 1, from 20 to 60% of an abrasive,from 5 to 12% or an anti-tearing compound selected from the groupconsisting of boric acid and sodium metasilicate, and up to 40% of acolored ceramic pigment.

14. Aluminum metal coated with a vitreous enamel in accordance withclaim 5.

15. Aluminum alloy coated with'a vitreous enamel in accordance withclaim 5.

16. Aluminum metal coated with a vitreous enamel in accordance withclaim 6.

17. Aluminum alloy coated with a vitreous enamel in accordance withclaim 6.

Y References Cited in the file of this patent

1. A VITREOUS ENAMEL COMPOSITION ESPECIALLY ADAPTED FOR USE OF APROTECTIVE OR DECORATIVE COATING FOR ALUMIUM AND ALUMINUM BASE ALLOYSAND ADAPTED TO BE FIRED AT A TEMPERATURE WITHIN THE RANGE OF FROM ABOUT900* TO ABOUT 1000*F. CONSISTING ESSENTIALLY OF FROM ABOUT 0.5 TO ABOUT10% ZINC OXIDE, FROM ABOUT 5 TO ABOUT 25% POTASSIUM OXIDE, FROM ABOUT 5TO ABOUT 25% SODIUM OXIDE, FROM 0 TO ABOUT 5% LITHIUM OXIDE, FROM 15 TOABOUT 30% SILICA, FROM ABOUT 15 TO ABOUT 25 TITANIUM DIOXIDE, FROM ABOUT1 TO ABOUT 15% BORIC OXIDE, FROM 0 TO ABOUT 25% PLUMBOUS OXIDE AND FROMABOUT 3 TO ABOUT 13% ANTIMONY TRIOXIDE, THE LEVEL OF COMBINED SODIUMOXIDE, POTASSIUM OXIDE AND LITHIUM OXIDE COMING WITHIN THE RANGE OF FROMABOUT 22.5 TO ABOUT 37.5%, AND THE SAID WEIGHT PERCENTAGES BEING BASEDUPON THE COMPOSITION OF THE ENAMEL AFTER FIRING.